In 2003, Kiselev et al. observed that when a spin-polarized direct current (DC) flowed through a giant magneto-resistance (GMR) multilayer of nano-size, it would develop a spin transfer torque (STT) that could oscillate magnetization of a free layer included in the GMR multilayer, thereby generating a high frequency output signal under certain conditions (see Kiselev S I, Sankey J C, Kirvorotov I N, et al. Microwave Oscillations of a Nanomagnet driven by driven by a Spin-polarized Current. Nature, 2003, 425:380). This phenomenon has been utilized to make a spin torque oscillator (STO). The spin torque oscillator has many advantages such as a simple structure, a smaller size (one fiftieth or so of a prior art crystal oscillator), a wide range of frequency modulation (0.1-100 GHz), ease of integration, a lower operating voltage (<0.5V), and the like. The spin torque oscillator has successfully overcome many problems associated with conventional LC oscillators and crystal oscillators and thus it is deemed as a candidate of the next generation of oscillators and being researched widely.
Though, the spin torque oscillator has its own defects, i.e., a relatively low output power. The output power of the spin torque oscillator is proportional to a square of magnetoresistance of a GMR spin valve element or a magnetic tunnel junction (MTJ) element constituting a core part of the spin torque oscillator, and now even a spin torque oscillator made of a MTJ element having a relatively high magnetoresistance has only an output power on the order of nanowatt (nW), which is much lower than the order of milliwatt (mW) required for practical use. In 2013, Zeng Z M et al. achieved an output power up to 63 nW with a novel magnetic tunnel junction configuration (see Zeng Z M, Finocchio G, Zhang B, et al. Ultralow-current-density and bias-field-free spin-transfer nano-oscillator. Sci Rep, 2013, 3:1426). However, this is still far below the output power requirement of the spin torque oscillator desired for practical use.